Project Summary All typically developing infants develop language, yet significant disparities exist across infants of different backgrounds in vocabulary growth starting as early as infants' first year. Prior research has documented the importance of input and caregiver-infant social interactions for language development; however, the specific mechanisms that support developmental change are unclear. One clearly established finding is that caregiver responsiveness has effects on language outcomes. Specifically, sensitive responsiveness (i.e., following in to infants' attentional focus) facilitates development, whereas redirective responsiveness negatively impacts development; however, a recent study found that redirective responses were positively associated with language development if the response was successful in re-orienting infants' attention. Thus, it needs to be determined how infants' attentional behavior changes in the moment in response to different caregiver response styles and relate these immediate effects to word learning and language outcomes. Therefore, the long-term objective of the study is to uncover processes underlying the association between caregiver responses and language development, which is important to identify possible interventions that can support robust language development in all infants. The specific aims of this study are to: 1) determine real-time changes in infant visual attention as a result of mothers' and fathers' interactive and response styles; and 2) link infant visual attention during social interactions to word learning and language development. This study will evaluate whether visual attention mediates the association between caregiver responses and language outcomes. For Specific Aim 1, the study will make use of novel technology, head-mounted eye-trackers. Caregivers and infants will engage in freeplay interactions in the lab while infants are wearing a head- mounted eye-tracker. Experimental manipulation of parents' responses to infant vocal and nonvocal exploratory behavior will elucidate the effects of sensitive vs. redirective responses on infants' real-time visual attention when infants are 10 months of age. For Specific Aim 2, word learning and language will be assessed at 14 and 18 months of age. The relationship between response types, infant visual attention, and vocabulary will be explored. An important outcome of the research linking caregivers' responsiveness and infants' visual attention to variation in vocabulary growth will contribute to understanding individual differences in language development. Furthermore, by determining specific caregiver behaviors that are associated with infants' visual attention and learning will help characterize the infant-caregiver system that drives positive outcomes. Therefore, results will contribute to developing intervention and parenting programs for infants who are at risk for language delays, such as infants from low socioeconomic backgrounds, infants with cochlear implants, and infants with siblings with autism spectrum disorder.